Smoke, heat and combustible gas detectors are of several types that are generally very different in operation.
Ionization smoke detectors, whose functioning is related to the changes of the electric conductivity of the air in the presence of fire aerosols, react under the effect of combustion products in the form of visible or invisible fire aerosols.
According to the operating principle of these detectors, an electric field is set up between two electrodes by a DC voltage. When the air between the electrodes is ionized, for example by a radioactive source, the resulting ions move under the effect of the field in the direction of the oppositely charged electrode. An electric current results whose current strength depends on the number and velocity of the ions. When fire aerosols penetrate into the space between the two electrodes, a part of the ions is deposited onto the particles of these aerosols, up to about a thousand times heavier than the ions. The heavy ions thus obtained hardly move any longer because of the mass inertia and thereby no longer contribute to carrying of charges, and the current diminishes.
Combustible gas detectors consist of a semiconductive gas sensor selectively sensitive to certain combustible gases such as methane, propane and hydrogen.
In optical smoke detectors, the very sensitive optoelectronic system, being made up of a specially developed semiconductive diode as an infrared light source, a silicon cell as light receiver, a light absorber and a protection from light from other sources, functions according to the light diffusion principle. The optics of the diode emits light impulses in the shape of a hollow cone. As long as the measuring chamber does not contain any smoke, the infrared light does reach the receiver placed in the center of the axis of the light cone. In penetrating the labyrinth chamber, the smoke particles disperse the light rays in all directions. A part of the dispersed light reaches the photoelectric receiver which then produces an electric signal.
There is now no satisfactory product for checking the functioning of these three types of detectors.
Mixtures of dichlorodifluoromethane and diclorotetrafluoroethane in proportions of 80 and 20% by weight, respectively, offered in aerosol cans are now packaged to check the functioning of ionization smoke detectors. These mixtures are not suitable for other types of detectors.
There is now no satisfactory product for checking the functioning of all three types of detectors. The nature of the products to check the correct functioning of each type of detector is different, depending on the type of detector used.
For ionization smoke detectors, products are used that stop the ions created in the air by the radioactive source, particularly by the formation of sufficiently heavy particles, and of sensing radioactive source radiations. These products can be organic chlorofluoro, fluoro products and gases such as nitric oxide and carbon dioxide.
The combustible gas detectors respond selectively, depending on the nature of the hydrocarbon; on the other hand, they are all more sensitive to other products such as alcohols, ethers, aldehydes, ketones, and, generally, to numerous volatile organic compounds.
Two means can be used for optical smoke detectors to create particles diffracting the light. According to one method, an aerosol of fine liquid droplets, of products with a slight vapor tension less than 0.1 millibar, and preferably less than 0.01 millibar, dissolved in a large amount of a solvent with a high vapor tension, can be used. According to another technique, a liquid which, on vaporizing, sufficiently cools the atmosphere of the chamber to transform the steam of the air into ice crystals, can be used.